Known vehicle seat slide device includes a pair of lower rails fixed to a vehicle floor, a pair of upper rails fitted into the lower rails to be slidable in the longitudinal direction relative to the lower rails and supporting a seat, and a lock device for maintaining each lower rail and upper rail to be fixed. Japanese Patent Laid-Open Publication No. H11-348627 discloses a known vehicle seat slide device which includes a pair of lower rails having U shaped cross section and a pair of upper rails having approximately reversed T shaped cross section. In the construction of the known vehicle seat slide device according to Japanese Patent Laid-Open Publication No. H11-348627, a bottom portion of the upper rail is inserted into the lower rail and engages with opposing side wall portions of the lower rail via rollers and shoes. A plurality of lock bores are provided on one of the opposing side wall portions of the lower rail in series with a predetermined distance one another. An engaging piece of a lock lever pivotally supported by a bracket fixed to the upper rail is inserted into lock bores provided on the bracket, the side wall portion of the lower rail, and the bottom portion of the upper rail. Under a state that the engaging piece of the lock lever is inserted into the lock bores of the bracket, the sidewall portions of the lower rail, and the bottom portions of the upper rail, the lock lever is supported on two points thereof by the upper rail side. And the lock lever opposes to the lock bore of the lower rail at a portion between the two supported portion thereof. Thus, when the impact force is applied to the seat upon the frontal or rear collision of the vehicle, shearing stress is generated on both of contact portions between the lock lever and the lock bore of the lower rail.
The lock lever is made of a plate member having an L shape in cross section, and a horizontal plate part of lock lever is the engaging piece which includes either two pawls or three pawls. The pawls are inserted into each lock bore. Thus, when the impact is applied on the seat, the horizontal plate part hits on an upstanding plate part of the lock bore of the lower rail, and the sharing force is generated on and shears the upstanding plate part. The impact force is absorbed by the shearing and mitigated effectively, and the drop out of the seat from the lower rail is prevented.
Because the strength of the upstanding plate part of the lower rail having the lock bores is determined to be lower than the strength of the plane plate of the lock lever, the engaging piece which is the horizontal plane plate of the lock lever is consecutively entering adjacent lock bores while breaking the upstanding plate portions of the lower rail to absorb the impact energy.
In the configurations of H11-348627, the lock bores of the lower rail are only provided on portions necessary for adjusting the seat position in the longitudinal direction of the vehicle. This prevents the undesired lock of the seat at the undesired position and further ensures the strength of the opposing sidewall portions of the lower rail. Notwithstanding, when the engaging piece of the lock lever is inserted into either at a front most or rear most position of the lock bore, when the horizontal plate portion serving as the engaging piece of the lock lever is broken by the impact force, the seat posture may suddenly and significantly be changed, because there is no adjacent lock bores to absorb the impact applied to the seat either by the collision from the front or the collision from the rear.
A need thus exists for a vehicle seat slide device which securely absorbs the impact due to the collision from the longitudinal directions.